Chapter 25, Sadava The History of Life on Earth • Many evolutionary changes take place over long periods of time. • To study long-term evolutionary change, we must think in time frames spanning millions of years, and imagine conditions very different from today’s. • Fossils are preserved remains of ancient organisms. They tell us about body form or morphology, and where and how the organisms lived • Earth’s history is recorded in rocks • Layers of rocks are called strata. • Relative ages of rocks can be determined by looking at strata of undisturbed sedimentary rock (formed by the accumulation of grains on the bottom of bodies of water). • The oldest layers are at the bottom, youngest at the top. • First observed in the 17th century by Nicolaus Steno. • In the eighteenth century, geologists realized that fossils could also be used to age rocks • Certain fossils were always found in younger rocks, others were found in older rocks • Fossils in more recent strata were more similar to modern organisms • Radioisotopes found in the fossils can be used to age them • Radioisotopes can be used to determine the actual age of rocks • Radioisotopes decay in a predictable pattern. • Elements can exist as different isotopes – different # of neutrons • Half-life is the time interval over which one half of the remaining radioisotope decays, changing into another element. • Used to date igneous rocks and metamorphic rocks • Fossils found in these rocks are determined to be the same age as the rock Sedimentary rock is made up of material from other rocks that have been weathered, broken, and often transported. Therefore, sedimentary rocks are aged by aging the actual fossils found there Element for aging is Carbon 50,000 years or younger • In an organism, the ratio of during its lifetime • 14C 14C to 12C stays constant is an unstable isotope of carbon • Would have vanished from Earth’s atmosphere if not for influx of cosmic rays • Therefore, a living organism incorporates 14C in its body from the environment • When an organism dies, it is no longer incorporating 14C from the environment • There is no replacement of 14C and the ratio of 14C to 12C decreases in the body • The dead organism is buried under sediment and debris over time • This ratio can then be used to date fossils up to about 50,000 years old • works in sedimentary rock • Isotopes in sedimentary rock cannot be used because the material making up the rock has come from various places and times • Dating rocks older than 50,000 years requires estimating isotope concentrations in igneous rocks (formed when molten material cools) • Decay of potassium-40 to argon-40 is used What period are we in now? Dinosaurs went extinct ~65mya • The idea that land masses have moved over time was first suggested by Alfred Wegener in 1912 • By the 1960s, evidence of plate tectonics convinced geologists that he was right • Earth’s crust is divided into solid plates about 40 km thick—collectively, the lithosphere • The plates float on a fluid layer of liquid rock or magma • Heat from radioactive decay in Earth’s core causes the magma to circulate in convection currents. This exerts pressure on the plates and causes them to move. • The movement of plates is called continental drift • Where plates are pushed together, they move sideways past one another, or one is pushed underneath the other • Mountain ranges are pushed up, and deep rift valleys or trenches are formed • Where plates are pushed apart, ocean basins form. • Position of the continents has changed dramatically over time. • Position and size of land masses influences ocean circulation patterns, sea level, and global climate. • Mass extinctions of marine animals have occurred when sea level dropped, exposing the continental shelves. • Earth’s atmosphere has also changed • The early atmosphere probably contained little or no free oxygen (O2) • O2 began to increase when certain bacteria evolved the ability to use H2O as a source of H+ ions in photosynthesis. O2 was a waste product. • Cyanobacteria formed rocklike structures called stromatolites which are abundant in the fossil record • Enough O2 was liberated to allow evolution of oxidation reactions as the energy source to synthesize ATP • When O2 first appeared in the atmosphere it was poisonous to some of the anaerobic prokaryotes • Some evolved the ability to metabolize the O2 • Advantages: Aerobic metabolism is faster and more energy is harvested. • Aerobes replaced anaerobes in most environments. • Atmospheric O2 also made possible larger and more complex cells. • About 1.5 billion years ago, atmospheric O2 concentrations became high enough for large eukaryotic cells to evolve. • Further increases in O2 levels 750 to 570 million years ago (mya) enabled evolution of multicellular organisms. Figure 25.5 Larger Cells, Larger Organisms Need More Oxygen • O2 concentrations increased again during the Carboniferous and Permian periods because of the evolution of large vascular plants • Extensive burial of plant debris in swamps formed coal deposits • The buried organic material was not subject to oxidation, and the living plants were producing large quantities of O2 • O2 levels were about 50 percent higher than today’s levels. • It allowed evolution of giant flying insects and amphibians that could not survive in today’s atmosphere. Figure 25.6 Rising Oxygen Levels and Body Size in Insects • Many physical conditions have oscillated over time in response to drifting continents, volcanic activity, and even extraterrestrial events such as meteorite impacts. • Sometimes these events caused mass extinctions in which a large proportion of the living species disappeared. • Earth’s climate has changed over time • Sometimes Earth was considerably hotter than today; sometimes colder, with extensive glaciation. • The cold periods were separated by long periods of milder climates • Major climatic shifts have occurred over periods as short as 5,000 to 10,000 years, primarily as a result of changes in Earth’s orbit around the sun. • Some climate changes have been even more rapid. Extinctions caused by them appear to be “instantaneous” in the fossil record. • The earth is in a warming trend now. HOWEVER, it is happening rapidly • Today’s rapid climate change is thought to be due to increasing CO2 concentrations, mostly from burning fossil fuels. • We are reversing the process of organic burial that occurred in the Carboniferous and Permian, but over a few hundred years rather than the millions of years over which these deposits accumulated. • The current rate of increase of atmospheric CO2 is unprecedented in Earth’s history. • If CO2 concentration doubles, average Earth temperature will increase, causing droughts, sea level rise, melting ice caps, and other major changes. • Collisions with large meteorites are probably the cause of several mass extinctions. • Dinosaurs ~65mya? • Evidence of impacts include large craters and disfigured rocks; compounds in the rocks with helium and argon isotope ratios characteristic of meteorites. • The assemblage of all kinds of organisms alive at one time (or in one place) is called the biota. • Flora – plants • Fauna - animals • Although about 300,000 species of fossils have been described, they are only a tiny fraction of all the species that have existed on Earth. • Only a tiny fraction of organisms become fossils, and only a fraction of those are studied by paleontologists. • Most organisms are decomposed quickly after death. • If they are transported to sites with no oxygen, where decomposition is very slow, fossilization could occur. • Many geologic processes transform rocks and destroy the fossils they contain, or bury them too deeply to be accessible. • A large number of fossil species are marine organisms that had hard shells or skeletons that resist decomposition. • Insects and spiders are also well represented in the fossil record.